Area Forecast Discussion
Issued by NWS Grand Rapids, MI

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FXUS63 KGRR 281723
AFDGRR

Area Forecast Discussion
National Weather Service Grand Rapids MI
123 PM EDT Fri Apr 28 2017

LATEST UPDATE...
Aviation

.SYNOPSIS...
Issued at 330 AM EDT Fri Apr 28 2017

A system passing south of Michigan is expected to bring some
showers to the area late this afternoon into early tonight. Most
of the rain should be near and south of Interstate 96. There will
then be a break in the rainfall during the morning hours of
Saturday. A storm from the southern plains will bring rain back
into the area by Saturday evening then rain with scattered
thunderstorms is expected into Monday morning.

&&

.SHORT TERM...(Today through Sunday)
Issued at 330 AM EDT Fri Apr 28 2017

There are two periods of rain we are watching. The more
significant event is Saturday night into Monday but we do have a
system passing south of Michigan this evening. We are in the jet
entrance region so we may see some rain from this but the best
instability is south of Michigan and this were most of the
rainfall is expected. I have only chance pops of this event.

The main rainfall event starts Saturday night and continues
through Sunday. We have system that digs into the southwest then
closes off and heads toward the Great Lakes for Saturday night
into Sunday. The problem is the upper jet does not allow the warm
front to come through so we end up having the low level jet and
1000/850 mb moisture transport vectors pointed at Southwest
Michigan Saturday night into Sunday night. The precipitable water
values are 2 to 3 standard deviations from normal based on the
Western Region Ensemble page (near 1.5") Sunday. We do get a good
coupling of upper difference and surface convergence plus the
model sounding are saturated from near 950 mb to 200 mb with only
small cape. This is all classic for heavy rain so I do expect
Southwest Michigan to get 1 to 3 inches of rain by Sunday evening.
The strong thunderstorms should largely stay south of the state
but as the occluded front comes through Sunday evening that will
have to be watched.

The models continue to be very consistent with this event so I do
believe it is more likely than not that we will see over 2 inches
of rain between Sat evening and Sunday night.

.LONG TERM...(Sunday night through Thursday)
Issued at 330 AM EDT Fri Apr 28 2017

The long term period is looking chilly and wet. Other than a near
normal temperature day Monday, high temperatures will be 5 to 10
degrees below normal.

The deep low that will produce quite a bit of rainfall will still be
west of the cwa Sunday night. Both the gfs and ecmwf show show the
warm front near the MI/IN border. A llj intersecting the front will
produce showers/storms and potentially heavy rainfall Sunday night
into Monday morning. The ecmwf in particular shows another 1.5
inches of rain after 00z Monday across the southern 2/3 of the cwa.
This will certainly cause rivers to rise, especially given the rain
that is expected to fall prior to Sunday evening.

Rain chances will diminish Monday afternoon as the occluded front
pushes east and we get into the dry slow. Pops go up a bit Monday
evening as wrap around moisture advects in from the west courtesy of
the upper low and then back down Tuesday with dry wx expected
Wednesday and Thursday. &&

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 123 PM EDT Fri Apr 28 2017

VFR conditions will continue until mid day Saturday despite
scattered showers passing through this evening. Southwest winds
will remain around 10 knots, but drop off by early evening. The
winds will become northeast by Saturday morning and increase to
around 10 knots again.

Another surge of moisture and showers will begin to lower
conditions from the south toward noon Saturday. Expect the I-94
TAF sites will dip into the MVFR range, while the I-96 TAFs see
the VFR persisting through 18Z.

As an outlook, expect all areas will see a downward trend to IFR
or even LIFR into Saturday afternoon and evening as rich low
level moisture arrives. Isolated thunderstorms may be possible by
evening toward I-94.

&&

.MARINE...
Issued at 330 AM EDT Fri Apr 28 2017

Looking at the latest observations along the coast and the buoy
data we were still getting waves around 3 1/2 feet but the winds
and the waves continue to decrease so I allowed the Small Craft
Advisory headlines to expire at 2 am. While the winds do increase
from the northeast Saturday, due to the easterly component of the
wind waves should stay below 4 feet. So I would think we would not
need another small craft advisory until at least Monday when
winds become more westerly again.

&&

.HYDROLOGY...
Issued at 334 PM EDT Thu Apr 27 2017

All major medium range models continue to indicate a solid swath of
rainfall across our region late this weekend, with 1"-3" or more
expected. The heaviest amounts may fall across the central and
western reaches of the Muskegon, Grand, and Kalamazoo River basins.
We are expecting within bank rises at the least and a possibility of
flood stage being reached at some locations. If 4" or 5" ends up
falling across portions of these basins then a more significant
flood threat would result with even greater impacts.

There is about a 36 hour window when the vast majority of this rain
will fall from Saturday night into Monday morning. Small rivers and
streams may rise abruptly so residents along these types of
waterways should monitor closely throughout the day Sunday and
Monday in particular.

&&

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...None.
LM...None.
&&

$$

SYNOPSIS...WDM
SHORT TERM...WDM
LONG TERM...04
AVIATION...JK
HYDROLOGY...Hoving
MARINE...WDM



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